In the measuring mechanism or the drawing mechanism of a laser printer, an optical scanning system is employed. The optical scanning system makes a beam emitted by a light source such as a laser diode pass through a condenser lens, deflects the beam by a deflector such as a polygon mirror or a swing mirror, and makes the beam form an image on the image plane of the scanning beam. In the optical scanning system for a laser printer, for example, it is important to keep the beam spot diameter constant for the whole scanned area (along the whole length of the scanning line).
Conventionally, in order to realize a constant beam spot diameter along the whole length of the scanning line, a long-length imaging lens is commonly arranged between the deflector and the image plane. However, the long-length imaging lens is not only expensive, but also requires a larger space for arrangement, and therefore an apparatus containing the scanning optical system is upsized.
Under the above-described situation, an optical scanning system which does not use a long-length imaging lens and uses a variable-focus element, which is capable of changing the focal point position in synchronization with scanning has been proposed. Refer to JPH02-293809A, for example. As variable-focus elements, a variable-focus element which is configured to change the refractive index of an electro-optic crystal by applying voltage across it, a variable-focus mirror which is made by making a surface of a piezoelectric bimorph a mirror surface, a variable-focus lens which is configured to electrically control a shape of an interface between two kinds of liquids which have different refractive indexes, a mechanism for directly moving a lens by the use of a linear motor, a piezoelectric element or the like, and so on are known. Refer to JPS61-185716A, for example.
The spot diameter ω′ at the intersection point between the image plane of the scanning beam and the principal ray on the plane perpendicular to the principal ray has the following relationship with wavelength λ of the light source and the numerical aperture NA.
                              ω          ′                ∝                  λ          NA                            A      When the beam is made to form an image on the image plane of the scanning beam by a variable-focus element, the distance along the beam path from the deflector to the image point at the center of the scanning line is smaller than the distance along the beam path from the deflector to the image point at each end of the scanning line. Accordingly, the numerical aperture NA (center) at the center of the scanning line is greater than the numerical aperture NA (end) at each end of the scanning line. As a result, the spot diameter ω′ at the image point at each end of the scanning line is greater than the spot diameter ω at the image point at the center of the scanning line, and therefore quality of drawing will deteriorate.
In order to prevent the spot diameter from becoming greater at each end of the scanning line as described above, an optical system in which a variable-focus lens system and a fixed-focus lens system are arranged a distance away from each other, the distance corresponding to the focal length of the fixed-focus lens system, has been proposed. Refer to JPH07-225349A, for example.
In this conventional optical system, however, a variable-focus optical system in which a number of fixed-focus lenses and a number of variable-focus lens are combined besides the imaging lens is employed, and therefore the optical system requires a relatively complicated structure and high costs. In the optical system, there is an idea that the fixed-focus lenses described above are also used as the imaging lens for the image plane of the scanning beam for a simpler structure. However, in this case, the focal length of the fixed-focus lenses becomes greater, and the distance between the variable-focus lens system and the fixed-focus lens system has to be increased. As a result, the apparatus is upsized.
In the above-described optical system, the spot diameter a is kept constant independently of the angle of deflection. However, considering that the beam is obliquely incident onto the image plane of the scanning beam at the incident angle of φ, the spot diameter ω on the image plane of the beam scanning, which exerts a direct influence upon the quality of drawing can be expressed by the following expression.
                    ω        =                                            ω              ′                                      cos              ⁢                                                          ⁢              ϕ                                ∝                      λ                          NA              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              ϕ                                                  B      Accordingly, in order to keep the spot diameter ω on the image plane of the beam scanning constant along the whole length of the scanning line, the numerical aperture NA (end) at each end of the scanning line has to be made greater than the numerical aperture NA (center) at the center of the scanning line.
Conventionally, a compact optical scanning system in which the spot diameter ω on the image plane of the beam scanning can be kept constant along the whole length of the scanning line has not been developed.